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BDS2 CDS DMS > metals & alloys > Flashcards

metals & alloys Flashcards

1
Q

what are metals and alloys used for in dentistry?

A 
RPD framework
crowns
denture base
orthodontic appliance
restorations
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2
Q

what is the main drawback of metals?

A

poor aesthetics

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3
Q

what is the advantage of metals?

A

superior mechanical properties

  • strength
  • rigidity
  • hardness
  • elastic limit
  • some ductility
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4
Q

what is a metal?

A

aggregate of atoms in crystalline structure

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5
Q

what is an alloy?

A

combination of metal atoms in crystalline structure

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6
Q

what is ductility?

A

amount material will deform without fracture - subject to tensile stress

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7
Q

what property is demonstrated if compressive stress applied?

A

malleability

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8
Q

what does the crystalline structure of a metal depend on?

A

history- method of production

shaping- crucial for dental applications eg. cold working

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9
Q

which factors affect mechanical properties?

A

crystalline structure
grain size
grain imperfections

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10
Q

what are the 3 lattice arrangements?

A

cubic, face-centred cubic, body centred cubic

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11
Q

how are grains formed?

A 
molten state above melting point
cools
changes state 
plateau- liquid and solid
solid cools
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12
Q

describe crystal growth

A

first atoms act as nuclei of crystallisation
crystals grow to form dendrites
crystals (grains) grow until they impinge on other crystals
region where grains make contact called grain boundary

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13
Q

what is it called if crystal growth is of equal dimension in each direction?

A

equi-fixed grains

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14
Q

what is radical grain structure?

A

molten metal cooled quickly in cylindrical mould

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15
Q

what is fibrous grain structure?

A

wire pulled through die (narrow circular aperture)

-cold worked metal/alloy

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16
Q

what is the result of fast cooling (quenching)?

A

more nuclei

small fine grains

17
Q

what is the result of slow cooling on crystal growth?

A

few nuclei

large coarse grains

18
Q

what are nucleating agents?

A

impurities or additives act as foci for crystal growth

19
Q

what is a grain boundary?

A

change in direction of the crystal planes

20
Q

why are small grains advantageous?

A

high elastic limit

increased FS and UTS, hardness

21
Q

what is the disadvantage of small grains?

A

decreased ductility

22
Q

what are the factors needed to quench?

A

small bulk
heat metal/alloy just above melting point before cooling
store in mould/container that readily conducts heat

23
Q

how do defects affect crystal/grain?

A

weaken grain so overall structure
grain not perfect
bond less effective than perfect ones
if force applied, defect more likely to be fractured

24
Q

what is the result if force applied to defect?

A

defect moves along lattice plane

  • crystal shape changes
  • characteristics of metal change
  • until reaches grain boundary so grain has all atoms inter-connected with same strong bond
25
Q

what effect does impeding movement of dislocations have on metals/alloys

A 
increases:
-elastic limit
-fracture stress/UTS
-hardness
decreases:
-ductility
-impact resistance
26
Q

what factors impede dislocation movement

A

grain boundaries
alloys-dif atom sizes
cold working
-dislocations stopped at grain boundaries

27
Q

what is cold work?

A

change shape by bending rolling etc.
done at low temp (below recrystallisation temp)
causes slip so dislocations collect at grain boundaries
so stronger, harder material

28
Q

what does cold work increase?

A

elastic limit
fracture stress/UTS
harness
residual stress

29
Q

what does cold work decrease?

A

ductility
impact strength
corrosion resistance

30
Q

what is the impact of residual stress?

A

causes instability in lattice
results in distortion over time
undesirable

31
Q

what is residual stress relieved by?

A

annealing

32
Q

what is annealing?

A

heating m/a so that greater thermal vibrations allow migration of atoms
re-arrangement allows any instability/distortion w/i lattice to be eliminated

33
Q

when is recrystallisation used?

A

when cold working hasn’t had desired effect?

34
Q

what is recrystallisation?

A 
m/a heated causing:
-smaller, equiaxed grains
-lower EL, UTS, hardness
-increased ductility
re-set
allows further cold work
can repeat until correct shape obtained
35
Q

what determines recrystallisation temp?

A

depends on amount of cold work

greater cold work-> lower temp needed

36
Q

what effect does excessive temp rise cause on grain growth?

A

large grains replace smaller course grains yielding poorer mechanical properties

BDS2 CDS DMS (16 decks)

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